It conveniently assumes that it's possible to change the charges on such
a time-scale without the resulting magnetic fields totally negating the
effect.

A full analysis will show either that there's no net force, or that the
change in momentum is matched by the momentum carried off as a result of
a net radiation of electromagnetic waves in the opposite direction.

Oh, and the fact that this thing will inevitably radiate electromagnetic
waves means that it requires a constant input of power.

Sylvia.

If one assumes that the principle of action and reaction can be overcome in electrodynamics it must accept the consequences also on the law of inertia..
And the change of the law of inertia is a great opportunity to shorten the time of interstellar travel